Various measurement devices for measuring electrical parameters (such as voltage, current and resistance) are widely used in industry and daily life. A multi-meter is a typical electronic measurement device, which is mainly used for measuring AC and/or DC voltages, currents and resistors. A typical multi-meter generally has a pair of test leads, each having one end connected to a subject apparatus or device and the other end inserted into a corresponding jack on a panel of the multi-meter, thus electrically connecting the subject equipment to the measuring circuit in the multi-meter.
Generally, a voltage difference (i.e., burden voltage) between two input terminals of a current sensing circuit needs to be maintained within a specific range to prevent the voltage difference from affecting the subject device. However, in conventional current sensing circuits such as a current sensing circuit shown in FIG. 1, a shunt resistor 11 is coupled between the two input terminals. Hence, it is difficult to use a large shunt resistor to obtain a better signal-to-noise ratio.
As shown in FIG. 1, the current sensing circuit 10 includes the shunt resistor 11, an amplifier 12, and a fuse 13. The shunt resistor 11 is connected in series with a subject device (not shown), thus converting the subject current IS into a voltage drop across the shunt resistor. Moreover, the amplifier 12 is configured as a voltage follower, with its non-inverting input node coupled to the shunt resistor 11 to sample the voltage drop, and its inverting input node coupled to an output node of the amplifier 12 to transfer the voltage drop to the output node. In the current sensing circuit 10, the fuse 13 is connected in series with the subject device and the shunt resistor 11, which is activated in case that the subject current IS exceeds a predetermined magnitude to prevent damage of the current sensing circuit 10 due to the excessive current.
However, for the current sensing circuit 10 shown in FIG. 1, the burden voltage (the voltage between the two input terminals coupled to the subject device) is equal to the sum of a voltage generated according to an input impedance (including the shunt resistor 11, the fuse 13, and a wire distribution resistance RW) of the current sensing circuit 10 and an input offset voltage of the amplifier 12. Apparently, the burden voltage of the current sensing circuit 10 increases with the subject current. Thus, the fuse 13 and the shunt resistor 11 in the current sensing circuit 10 should be as small as possible. However, it is difficult to obtain a high signal-to-noise ratio if the shunt resistor 11 having a small resistance is used in the current sensing circuit 10.
FIG. 2 shows another typical current sensing circuit 20. As shown in FIG. 2, the current sensing circuit 20 includes a shunt resistor 21, an amplifier 22, and a fuse 23. The shunt resistor 21 is coupled in a feedback loop of the amplifier 22, i.e., between an inverting input node and an output node of the amplifier 22. Thus, the shunt resistor 21 does not affect the burden voltage between the two input terminals of the current sensing circuit 20. The burden voltage is mainly determined according to the voltage drop across the fuse 23 and the wire distribution resistor, which are generated by the subject current flowing therethrough. However, the fuse 23 generally has a non-negligible resistance, and the voltage drop across the fuse 23 may significantly affect the burden voltage if the subject current is large. Thus, the current sensing circuit 20 still needs to use a fuse having a small impedance, therefore, it is not suitable for measuring large currents.